The Sabertootlns 



Repeat 

 Performances 



by Christine Janis 



The immense diversity of mammals liv- 

 ing today is the legacy of historical events. 

 One was the Cretaceous extinction of the 

 dinosaurs, which had monopoUzed most 

 of the earth's Uving space for millions of 

 years. With the dinosaurs gone, a major 

 constraint on mammalian evolution and 

 radiation was removed. But perhaps even 

 more important was the breaking up of the 

 supercontinent Pangaea, which began 

 some 130 million years ago and was still 

 going on in the early Cenozoic, about 65 

 million years ago, when the modem radia- 

 tion of mammals began. 



As the continents went their separate 

 ways, they carried their mammals with 

 them. Separated from other populations of 

 their kind, these "seed faunas" of small, 

 early mammals were free to evolve in iso- 

 lation. Each continental block developed 

 its own group of mammals. Placental 

 mammals, for example, probably origi- 

 nated in Asia, in the early Cretaceous. 

 Monotremes (the egg-laying platypus and 

 echidnas) were almost certainly native to 

 Australia. Other animals endemic to that 

 continent today — the marsupials — came 

 somewhat later, their ancestors crossing 

 over Antarctica from South America in the 

 early Cenozoic, before the final separation 

 of the southern continents. 



Sometimes continental blocks gained 

 mammals from ancestors that dispersed, 

 by chance, across the still-widening 

 oceans. In some cases, chance dispersal 

 added to an already existing fauna. In oth- 

 ers, however, it was the sole source. Mada- 

 gascar and New Zealand, for instance, ap- 

 parently broke away too early to have their 

 own mammal conringents, but while 

 Madagascar developed a rich mammalian 

 fauna by waves of dispersal. New Zealand 

 has no native mammals other than bats. 



The results of all this mammalian radia- 

 tion were widely diverse, but certain simi- 

 larities can be found among mammals that 



are only distantly related. Some of the sim- 

 ilarities — such as the production of milk to 

 feed the young — are attributable to the 

 shared ancestry of all mammals, but others 

 came about as long-separated types of 

 mammals independently discovered simi- 

 lar "solutions" to the "problems" posed by 

 similar environments. Examples of such 

 convergence can often be seen in the body 

 forms of mammals from different conti- 

 nents. Such forms are referred to as con- 

 vergent ecomorphological types. The term 

 ecomorph refers to the impact of behavior 

 and ecology on the evolution of an ani- 

 mal's anatomy. 



The most familiar examples of conver- 

 gence today are those of Australian marsu- 

 pials and placental mammals that evolved 

 elsewhere in the world: the thylacine, or 

 Tasmanian "wolf (believed by most peo- 

 ple to have gone extinct earlier this cen- 

 tury, although tantalizing reports of its 

 continued survival appear now and then), 

 and the wolf of the Northern Hemisphere, 

 for instance. A striking amount of conver- 

 gence can also be seen in the "flying" — 

 actually gliding — possums of Australia 

 and the two separate groups of placental 

 "flying" squirrels (one in the Northern 

 Hemisphere and one in Africa), as well as 

 Asia's so-called flying lemurs (not only do 

 fliey glide, they are also not true lemurs). 



Australia is a fruitful place to look for 

 examples of convergence because of its 

 long history of isolation. Madagascar, too, 

 has produced many native mammals con- 

 vergent with mammals elsewhere: the 

 fossa (a giant civet), almost indistinguish- 

 able from a cat; tenrecs, some of which re- 

 semble Northern Hemisphere hedgehogs 

 and moles; and lemurs, primates that radi- 

 ated into a wide variety of forms. Some 

 extinct giant lemurs appear to have been 

 ecomorphs of such animals as the marsu- 

 pial koala, the placental orangutan, and the 

 extinct placental ground sloths. 



In many other parts of the world, the 

 continuing shifting of the continents and 

 the migrations that follow the periodic 

 lowering of sea levels have blurred much 

 of the originally distinct character of the 

 various continental faunas and aimihilated 

 many unique forms. Africa was the first to 

 suffer, when it docked with Eurasia in the 

 late Oligocene or early Miocene, between 

 twenty and thirty million years ago. The 

 little rock hyrax, or coney, for example, is 

 the sole survivor of a great diversity of 

 hyraxes that once included piglike, hippo- 

 like, and antelopelike forms. 



North and South America have lost 

 much of their early mammalian diversity 

 as well. Hippolike rhinos and giraffelike 

 camels no longer roam the savannas of 

 North America, and five entire orders of 

 native ungulates — which evolved into 

 forms paralleling rhinos, horses, and 

 camels living elsewhere in the world — are 

 now extinct in South America. Enough 

 species remain to bear witness to each 

 continent's period of isolation, and plenty 

 of examples of convergence can still be 

 found — the South American armadillos 

 and African pangolins, for example — but 

 the faunal blending that has occurred over 

 time has reduced the opportunities to ob- 

 serve the phenomenon in living animals. 



The fossil record offers a chance to dis- 

 cover more instances of convergence. It 

 also provides an example of an evolution- 

 ary phenomenon that resembles classic 

 convergence, but that takes place over 

 time ratiier than over space. "Iterative" 

 evolution involves the appearance, extinc- 

 tion, and reappearance of the same eco- 

 morphological type, sometimes (but not 

 always) in die same taxonomic group. The 

 best-known and most dramatic examples 

 of iterative evolution are the saber-toothed 

 carnivores. The saber-toothed eco- 

 morph — a predator with elongated ca- 

 nines and a powerful body — has existed 

 several times in the past, which raises the 

 question of what factors may dispose cer- 

 tain types of animals to extinction. 



The most familiar of the sabertooths is 

 the Pleistocene saber-toothed tiger (Smilo- 

 don) of the La Brea tar pits in Cahfomia. 

 This animal was a true cat (family Felidae) 

 but was only distantiy related to the animal 

 we call tiger today. Saber-toothed cats first 

 appeared in the Old World in the later 

 Cenozoic, but the earliest version of a 

 saber-toothed mammalian predator, 

 Machaeroides, had evolved some fifty 

 million years before, in the Eocene, and 

 belonged to the extinct carnivorous order 

 Creodonta. Marsupial versions of saber- 



78 Natural History 4/94 



